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NOTICE: This Is the Author's Version of a Work That Was Accepted For NOTICE: this is the author’s version of a work that was accepted for publication in Carbon. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Carbon [VOL 50, ISSUE 3, March 2012] DOI 10.1016/j.carbon.2011.11.002 GUEST EDITORIAL Nomenclature of sp2 carbon nanoforms Carbon’s versatile bonding has resulted in the discovery of a bewildering variety of nanoforms which urgently need a systematic and standard nomenclature [1]. Besides fullerenes, nanotubes and graphene, research teams around the globe now produce a plethora of carbon-based nanoforms such as ‘bamboo’ tubes, ‘herringbone’ and ‘bell’ structures. Each discovery duly gains a new, sometimes whimsical, name, often with its discoverer unaware that the same nanoform has already been reported several times but with different names (for example the nanoform in Figure 1i is in different publications referred to as ‘bamboo’ [2], ‘herringbone-bamboo’ [3], ‘stacked-cups’ [4] and ‘stacked-cones’ [5]). In addition, a single name is often used to refer to completely different carbon nanoforms (for example, the ‘bamboo’ structure in [2] is notably different from ‘bamboo’ in Ref [6]). The result is a confusing overabundance of names which makes literature searches and an objective comparison of results extremely difficult, if not impossible. There have been several attempts to bring order to the chaos of carbon nomenclature. An IUPAC subcommittee on carbon terminology and characterization that ran until 1991 produced a glossary of 114 terms for carbon materials. However the only carbon nanoforms included were fullerenes* [7] and graphene† [8] (which remain the only carbon nanoforms defined in the IUPAC “Gold Book” [9]), and this was in any case a list of defined terms rather than an attempt to classify carbon forms based on underlying shape. Following an international symposium on nanocarbons in Japan in 2002, Inagaki and Radovic proposed a nanocarbon nomenclature based on preparation methods [10], drawing the important distinction between nano-structured and nano-sized carbon nanomaterials. However no definitions were given for detailed morphological differences. * Defined by IUPAC as “Compounds composed solely of an even number of carbon atoms, which form a cage-like fused-ring polycyclic system with twelve five-membered rings and the rest six-membered rings. The archetypal example is [60]fullerene, where the atoms and bonds delineate a truncated icosahedron. The term has been broadened to include any closed cage structure consisting entirely of three-coordinate carbon atoms.” doi:10.1351/goldbook.F02547 † Defined by IUPAC as “A single carbon layer of the graphite structure, describing its nature by analogy to a polycyclic aromatic hydrocarbon of quasi infinite size. The term Graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed.” doi:10.1351/goldbook.G02683 There are various ways that carbon nanoforms could be classified. These include their synthesis conditions, physical properties such as density and internal surface area, or graphitic sheet texture [11]. In the current editorial we attempt to develop a coherent nomenclature for carbon nanoforms based on their morphological differences and the geometrical transformations that relate one form to another. The structurally simplest carbon nanoform is graphene. Graphene is a near planar sheet of sp2– bonded carbon atoms in a hexagonal network. It can be considered as the final member of the series of fused polycyclic aromatic hydrocarbons, such as naphthalene, anthracene and coronene. Terms such as ‘single graphene layer’ and ‘graphene nanosheet’ are redundant since graphene is a layer/sheet, always single and necessarily in the nanoscale as it is one atom thick. The preferable term is simply ‘graphene’. Graphene can serve as the initial building block for a series of thought experiments, generating other carbon nanoforms by the application of geometrical transformations. Morphological variation in sp2-based carbon nanoforms is a consequence of curvature, introduction of structural defects, and dangling bonds. The result of this variation is deviation from ideal infinite planar- sheet aromaticity, with corresponding localisation of states and variation in chemical reactivity. We can classify an initial list of transformations as follows: Stacked: one-dimensional ordered arrangement of multiple nanoforms of the same type (e.g. from graphene to graphite). When a stack length is significantly larger than the dimensions of the individual nanoform, the term ‘fibre’ can be applied. Cut: breaking a line of C-C bonds, for example producing short segments from a longer nanotube via a cut perpendicular to the tube axis, or graphene nanoribbons from graphene. Circularly Wrapped: wrapping an object around an axis at a fixed distance, resulting in matching edges (e.g. from graphene to a single-walled nanotube). Scrolled: wrapping an object around an axis with continuously increasing distance, resulting in mismatched edges, i.e. spirally wrapped (e.g. from graphene to a graphene nanoscroll). 2 Coiled: object wrapped around an axis with an additional translation along that axis. Screwed: application of a screw dislocation along the periodic axis of a nanoobject or stack of nanoobjects. For 0D nanoobjects a screw dislocation dipole is required. This typically converts a stack of individual nanoobjects into continuous layers. Coned: removal of an in-plane wedge of material, reconnecting any resultant dangling bonds, resulting in pentagons at the apex (application of a wedge disclination). By applying the various transformations listed above to graphene we created a carbon nanoform “family tree” as shown in Figure 2. While the diagram is non-exhaustive (primarily for clarity), it helps to classify the nanoforms by morphology, and provides a first step towards a standardised nomenclature through use of these operations as adjectives to graphene. These transformations result in families of forms as follows (names are preferentially selected based on shape rather than analogy to everyday objects): Nanotube: commensurately wrapped two-dimensional nanoform, Nanoscroll: incommensurately wrapped two-dimensional nanoform, Nanotoroid: commensurately wrapped one–dimensional nanoform, Nanospiral: incommensurately wrapped one-dimensional nanoform, Nanocone: nanoform constructed from one or more stacked conical layers, Nanoribbon: layer subject to two parallel cuts resulting in a strip, Platelet: layer cut in at least two directions giving a patch (e.g. polyaromatic hydrocarbon) Fullerene: closed cage containing twelve pentagons*. The description of the nanoforms given thus far can be further refined through the use of additional terms, not included in the “family tree” since they can be applied to most of the forms already there: Single-/Double-/Multi-Walled: the wall of the nanoform consists of one, two, or multiple layers. Hollow / Full-core: nanoform contains (/ does not contain) an interior cavity space equal to or larger than twice the graphite inter-layer spacing. 3 Double-/Multi-cored: two or more nanoforms encapsulated in another nanoform of the same type. Partitioned: hollow-core nanoform with internal walls orthogonal to the object axis which divide the core into separate compartments. Corrugated: the layer surface undulates without changing the overall shape of the nanoform. The term applies when the scale of the undulations is less than the dimensions of the nanoform itself. Wavy: the nanoform itself undulates. Thus the undulations in a wavy nanoform are on a larger scale than in a corrugated nanoform. Curved: a curved nanoform is considered as a wavy nanoform where the period of the undulations is larger than the dimension of the nanoform Polygonised: layers are facetted, without continuous curvature. Collapsed: the distance between two otherwise separated points on a surface is reduced to approximately that of the inter-layer spacing of graphite (typically due to Van der Waals interaction). Vertical / Horizontal: orientation of the principal axis of the object with respect to a horizontal substrate. Bundled: an ordered group of multiple approximately parallel one-dimensional nanostructures (e.g. single-walled nanotube bundles). Crystallised: aggregate of nanoforms of the same type possessing long-range order (e.g. fullerite). Clustered: aggregates of nanoforms with no long- or short-range order besides that of the objects themselves. Closed / Open: object forms / does not form a continuous surface. Open / Closed tip/edge: Object has / does not have functionalised or dangling bonds at its tip/edge. For example closed-edge stacked platelets have carbon sheets which fold back on themselves, and an open-tipped nanotube has terminating functionalised bonds at its ends. 4 Certain terms used previously in the literature are replaced here. In this system for example the term ‘few layer graphene’ is incorrect and would be replaced by ‘few stacked graphenes’. For consistency with nanotubes, two and three stacked graphenes would be ‘double graphenes’ and ‘triple graphenes’ respectively. We propose the name ‘multi-walled fullerenes’ for multiply- encapsulated closed carbon layer structures (commonly referred to as “carbon
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